JAC Advance Access originally published online on March 8, 2007
Journal of Antimicrobial Chemotherapy 2007 59(5):854-859; doi:10.1093/jac/dkm060
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Extended double disc synergy testing reveals a low prevalence of extended-spectrum ß-lactamases in Enterobacter spp. in Vienna, Austria
1 Department of Clinical Microbiology, Institute of Hygiene and Medical Microbiology, Medical University Vienna, Vienna, Austria 2 Department of Hospital Hygiene, Institute of Hygiene and Medical Microbiology, Medical University Vienna, Vienna, Austria
* Correspondence address. Department of Clinical Microbiology, Institute of Hygiene and Medical Microbiology, Vienna General Hospital, Waehringer Guertel 18-20/5P, 1090 Vienna, Austria. Tel: +43-1-40400-5151; Fax: +43-1-40400-5228; E-mail: petra.apfalter{at}meduniwien.ac.at
Received 8 August 2006; returned 13 November 2006; revised 29 January 2007; accepted 5 February 2007
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Abstract |
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Objectives: The aims of this study were to determine the prevalence of extended-spectrum ß-lactamases (ESBLs) in AmpC-carrying Enterobacter spp. in a tertiary care university hospital in Vienna, Austria, and to implement a cost-effective strategy to detect ESBLs in this particular genus on a routine basis.
Methods: Clinical Enterobacter isolates (n = 208) were investigated by means of (i) an inhibitor-potentiated diffusion test using cefpodoxime, (ii) an expanded double disc diffusion synergy test (discs of cefotaxime, ceftazidime, cefpodoxime and cefepime placed around amoxicillin/clavulanic acid), (iii) the Etest ESBL screening method and (iv) the cefoxitin–cefotaxime antagonist test. Cefepime MICs were determined by separate Etests.
Results: Of 208 isolates, 76 (37%), 18 (9%) and 92 (44%) were derepressed, partially derepressed and inducible AmpC producers, respectively. Eight (4%) ESBL-producing Enterobacter strains could be detected, all of which would have been detected using disc-based tests. Six out of eight strains were genetically not related, as assessed by random amplification of polymorphic DNA. Typing results were confirmed by means of enterobacterial repetitive intergenic consensus PCR. The MIC90 of cefepime was not different in ESBL carriers (range 2–4 mg/L), and was especially low in inducible AmpC producers (0.125 mg/L). More than half of all Enterobacter isolates (n = 110; 53%) were partly derepressed or fully inducible AmpC producers. In the absence of cefoxitin, they appeared susceptible or intermediately susceptible to cefazolin (n = 8; 9%), cefuroxime (n = 75; 81.5%), ceftazidime (n = 91; 99%), cefotaxime (n = 92; 100%), cefpodoxime (n = 75; 81.5%) and cefepime (n = 91; 99%).
Conclusions: Susceptibility to third-generation cephalosporins would have been falsely assumed in more than half of all Enterobacter isolates, but ESBL in Enterobacter is currently rare in our institution. Integration of multiple double disc tests into the routine antibiogram seems a reliable approach to screen for emerging resistance mechanisms. Etests did not provide additional information in this study.
Keywords: Etest , AmpC , third-generation cephalosporins , fourth-generation cephalosporins
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Introduction |
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Enterobacter spp. can cause severe, difficult to treat nosocomial infections,1 and antimicrobial susceptibility testing of Enterobacter spp. can be challenging for numerous reasons. First, production of chromosome-encoded AmpC ß-lactamases is a typical mechanism of extended-spectrum cephalosporin resistance in Enterobacter spp.2 Although derepressed AmpC producers are easily detectable due to their in vitro resistance to third-generation cephalosporins, inducible AmpC-producing strains might be overlooked when susceptibilities are read in the absence of an inducer agent.3 Thus, susceptibility may falsely be assumed. Secondly, even though CLSI (formerly NCCLS) guidelines do not recommend screening Enterobacter spp. for extended-spectrum ß-lactamase (ESBL) production on a routine basis,4 ESBL-producing Enterobacter has been increasingly reported.5–8 Thirdly, both resistance mechanisms can occur simultaneously. In such situations, the use of clavulanic acid to inhibit an ESBL may induce high-level expression of the chromosomal AmpC enzyme and may then antagonize rather than protect the antibacterial activity of the partner ß-lactam, masking the synergistic effect required to detect ESBL production.9 Currently, there are no recommendations from CLSI for the detection of ESBL in AmpC ß-lactamase-producing Enterobacteriaceae.4
When administering empirical antimicrobial therapy, knowledge on local resistance mechanisms is essential. Although fourth-generation cephalosporins might be a therapeutic option in Enterobacter carrying only AmpC, they are not recommended in the case of an infection by ESBL-producing strains.10
The aims of this study were (i) to determine the prevalence of derepressed, partially derepressed and inducible AmpC as well as simultaneous ESBL production in Enterobacter strains in our institution, (ii) to evaluate our current strategy not to report results of second- and third-generation cephalosporins in the case of in vitro susceptibility, and indicating on the report that these agents are ‘clinically not indicated’, and (iii) to establish an easy and cost-effective procedure for the routine microbiology laboratory to detect ESBL in AmpC-carrying Enterobacter spp. on a routine basis.
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Materials and methods |
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Strains
Between October 2004 and March 2005, a random sample of 208 clinically relevant, non-duplicate Enterobacter isolates were routinely collected in the microbiology laboratory of the Vienna General Hospital and stored at –80°C using the Cryobank system (Mast Diagnostica, Reinfeld, Germany). All strains were obtained from hospitalized patients and were isolated from blood (n = 5; 2%), lower respiratory tract (collected by bronchoalveolar lavage; n = 48; 23%), urinary tract (n = 46; 22%), deep surgical wound infections (n = 73; 35%) and faecal swabs (n = 36; 17%) from bone marrow transplant recipients. The isolates were thawed and subcultured once over night at 35°C on Columbia agar supplemented with 5% sheep blood (Becton Dickinson, Heidelberg, Germany). Strains were retested for species identification using the VITEK-GN card, as recommended by the manufacturer (Vitek 2 system, BioMérieux, Marcy l'Étoile, France).
Antimicrobial susceptibility testing
Antimicrobial susceptibility testing was performed by means of the agar disc diffusion method following the respective CLSI (formerly NCCLS) guidelines.11 The following antimicrobial agents (all supplied from Oxoid, Hampshire, UK) were tested: ampicillin (10 µg), amoxicillin/clavulanic acid (20 µg/10 µg), cefazolin (30 µg), cefuroxime (30 µg), cefoxitin (30 µg), cefotaxime (30 µg), ceftazidime (30 µg), cefpodoxime (10 µg), cefpodoxime/clavulanic acid (10 µg/1 µg), cefepime (30 µg), imipenem (10 µg), gentamicin (10 µg), amikacin (30 µg), ciprofloxacin (5 µg) and trimethoprim/sulfamethoxazole (1.25 µg/23.75 µg).
ESBL production was tested by comparing the inhibitory zone diameter of a cefpodoxime disc and a cefpodoxime/clavulanic acid disc. A difference of 
5 mm in the zone diameter was considered as a positive result. This procedure is recommended by CLSI12 for ESBL screening in Klebsiella pneumoniae, Klebsiella oxytoca, Escherichia coli and Proteus mirabilis, and thus is integrated in the routine antibiogram tested for Enterobacteriaceae in our institution. For this study, all strains were additionally subjected to an expanded double disc diffusion synergy test. Preliminary experiments were performed to evaluate disc-to-disc distances of 20, 25 and 30 mm (centre to centre). A distance of 25 mm between discs was regarded as optimal to observe a ‘keyhole phenomenon’. This distance also allowed incorporation of double disc synergy testing into routine susceptibility testing with commercially available disc dispensers (Oxoid): discs of cefotaxime, ceftazidime, cefpodoxime and cefepime were placed around an amoxicillin/clavulanic acid disc at a distance of 25 mm, as described previously.13,14 A keyhole phenomenon was regarded as positive for ESBL production. In addition, all isolates were also tested by the Etest ESBL screening method using cefepime strips with and without clavulanic acid (AB Biodisk, Solna, Sweden).15 Etest reading and interpretation were carried out according to the manufacturer's instruction. Briefly, cefepime MIC reduction by three or more 2-fold dilutions with clavulanic acid, ‘phantom’ zones or deformation of the inhibition ellipse was indicative of ESBL production.
To study the inducibility of the AmpC enzyme, the cefoxitin–cefotaxime antagonist test was performed as described recently,16 and the ß-lactamase inducibility was confirmed by the presence of a blunted cefotaxime zone adjacent to cefoxitin. The latter test, as well as disc diffusion synergy testing, was done as displayed in Figure 1. Cefepime MICs were determined by means of separate Etest strips.
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Definition of resistance types
According to the characteristics of ß-lactamase production, resistance types were defined as follows:5 (i) Group 1, derepressed AmpC producers were resistant to cefoxitin (zone diameter 
14 mm), resistant or intermediate susceptible to cefotaxime (22 mm), had a negative cefoxitin–cefotaxime antagonist test and a negative ESBL production; (ii) Group 2, partially derepressed AmpC isolates were resistant to cefoxitin (14 mm), resistant or intermediate susceptible to cefotaxime (22 mm), had a positive cefoxitin–cefotaxime antagonist test and a negative ESBL production; (iii) Group 3, (partly) derepressed AmpC producers with ESBL production were resistant to cefoxitin (14 mm), had a negative cefoxitin–cefotaxime antagonist test and produced ESBL (see also Figure 1); (iv) Group 4, strains were susceptible to cefoxitin (18 mm) and had a positive ESBL production defined by an increase of 5 mm in the zone diameter of a cefpodoxime/clavulanic acid disc compared with the diameter of cefpodoxime alone, a keyhole phenomenon in the double disc synergy test to at least one of the broad-spectrum cephalosporins surrounding the amoxicillin/clavulanic acid disc and/or a positive cefepime/cefepime clavulanate Etest; and (v) inducible AmpC producers were susceptible to cefoxitin (18 mm), had a positive cefoxitin–cefotaxime antagonist test and a negative ESBL production.
Typing of ESBL-producing Enterobacter isolates
Random amplification of polymorphic DNA (RAPD) was applied for molecular typing. Enterobacterial repetitive intergenic consensus (ERIC)-PCR was performed for confirmation. The same DNA preparations were used for both assays, whereby DNA was extracted from pure cultures as described previously.17 RAPD was performed using the primer P15 (5'-AAT GGC GCA G-3').18 The PCR mixture included a Ready-to-Go RAPD Analysis Bead (Amersham Biosciences, Little Chalfont, UK), 2 µL (50 pmol) of primer P15, 3 µL of DNA solution and sterile distilled water for a final volume of 25 µL. The amplification conditions included an initial denaturation step (94°C for 5 min) followed by 40 cycles, each consisting of 94°C for 30 s, 36°C for 30 s and 72°C for 1 min.
For ERIC-PCR, the primer ERIC-2 (5'-AAG TAA GTG ACT GGG GTG AGC G-3') was applied.19 The amplification profile consisted of an initial denaturation of 4 min at 94°C, 35 cycles (94°C for 45 s, 58°C for 1 min and 72°C for 2 min) and a final elongation of 10 min at 72°C. The PCR products were separated by electrophoresis in 2% TopVisionTM LE GQ agarose (Fermentas Life Sciences, St Leon-Rot, Germany) for 3.5 h (2.5 V/cm) and visualized by ethidium bromide staining. The patterns were interpreted individually by two researchers, whereby single-band differences were ignored.20
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Results |
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Species identification of the 208 Enterobacter isolates collected for this study revealed 179 Enterobacter cloacae (86%), 27 Enterobacter aerogenes (13%) and one each Enterobacter sakazakii and Enterobacter asbinae. Susceptibility test results by means of agar disc diffusion were as follows: all strains were susceptible to imipenem; resistance rates for amikacin, gentamicin, ciprofloxacin and trimethoprim/sulfamethoxazole were 0.5% (1/208; 3 intermediate susceptible), 5.7% (12/208; 1 intermediate susceptible), 11.5% (24/208; 3 intermediate susceptible) and 11.5% (24/208; 7 intermediate susceptible), respectively.
Of 208 Enterobacter spp. isolates, 76 (37%), 18 (9%) and 92 (44%) were derepressed, partially derepressed and inducible AmpC producers, respectively. Fourteen strains could not be categorized (discussed subsequently). Thus, 95% (197/208) of Enterobacter isolates were definitely AmpC carriers.
All derepressed strains were resistant to cefuroxime and cefpodoxime, and >90% were resistant to cefotaxime (71/76) and ceftazidime (74/76). Only 3 out of 76 were also resistant to cefepime by disc diffusion, but none had an MIC >32 mg/L. In the group of the partially derepressed strains, one strain (1/18) was susceptible to cefuroxime and 40% strains were susceptible to cefotaxime (7/18) and ceftazidime (8/18). All strains were resistant to cefpodoxime and susceptible to cefepime.
Nearly half of all Enterobacter isolates (n = 92; 44%) were fully inducible AmpC producers. In the absence of cefoxitin, they appeared susceptible or intermediate susceptible to cefazolin (n = 8; 9%), cefuroxime (n = 75; 81.5%), ceftazidime (n = 91; 99%), cefotaxime (n = 92; 100%), cefpodoxime (n = 75; 81.5%) and cefepime (n = 91; 99%). Figure 2 summarizes all derepressed, partially derepressed and inducible AmpC producers with and without additional ESBL production with reference to their MICs of cefepime. The MIC90 of cefepime was not significantly different for ESBL-producing derepressed AmpC producers (4 mg/L) when compared with strains without ESBL production (2 mg/L), whereas for inducible AmpC producers, it was 0.125 mg/L. According to current CLSI breakpoints for Enterobacteriaceae, all isolates would have been in the susceptible range of cefepime.
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ESBL-producing Enterobacter spp
In the present study, eight (4%) ESBL-producing Enterobacter strains could be detected. Of these, seven were also fully or partly derepressed AmpC producers. The single cefoxitin-susceptible ‘ESBL only’ isolate probably was falsely defined as ESBL by the cefepime/cefepime clavulanate Etest due to a cefepime MIC reduction by three or more 2-fold dilutions. ESBL production could not be confirmed by the other methods used in this study. The concordance of test results for the other seven AmpC producers carrying ESBL is displayed in Table 1. In all strains with a significant MIC reduction by Etest in the presence of clavulanate (n = 4), an increase of 5 mm could be observed in the zone diameter of a cefpodoxime/clavulanic acid disc when compared with the diameter of cefpodoxime alone. All of the seven strains would have been detected by disc-based tests: six out of seven showed a keyhole phenomenon in the double disc synergy test to at least cefepime.
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The 14 strains that could not be categorized were confirmed as Enterobacter by GN and API 20 (BioMérieux). Seven of 14 were susceptible to cefoxitin, only 2 of 14 and 1 of 14 were resistant to cefazolin and cefuroxime, respectively, and none was resistant to cefotaxime, ceftazidime, cefpodoxime and cefepime. All ESBL tests were negative and MICs of cefepime were 0.125 mg/L in all 14 strains.
Genetic relationship between ESBL-producing Enterobacter isolates (n = 8)
Six different genetic profiles (Types A–F) could be determined and confirmed by RAPD and ERIC-PCR, respectively (data not shown). Strains 1 and 3 were genetically identical and assigned to Type A. These strains were isolated from different patients on different wards at different points in time. Strains 3 and 4 were also identical and assigned to Type C. The latter strains were isolated from very low birth-weight infant twins, who were nursed at the same neonatal intensive care unit at the same time. Thus, it is very likely that cross-contamination had occurred in this special case.
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Discussion |
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Enterobacter spp. can cause difficult to treat nosocomial infections in critically ill patients.1 Owing to mutations, the expression of a chromosomal gene encoding the AmpC ß-lactamase2 can be increased during therapy with third-generation cephalosporins such as cefotaxime, ceftazidime and ceftriaxone.3,21 As to what extent fourth-generation cephalosporins are affected by this mechanism is not fully understood.1,8,22–24 As of 2006, CLSI guidelines for antimicrobial susceptibility testing recommend to test repeated isolates of Enterobacter to detect developing resistance during prolonged therapy with third-generation cephalosporins, because initially susceptible strains may become resistant within 3–4 days after initiation of therapy. Whether this is also true for fourth-generation agents is not mentioned in the CLSI guidelines. In daily clinical routine, however, testing of more than one isolate is not always feasible and empirical as well as calculated antimicrobial therapy often is based on broad-spectrum cephalosporins. In addition, ESBL-producing Enterobacter has been increasingly reported.5–8 In this scenario, penicillins, aztreonam and all extended-spectrum cephalosporins should be reported as resistant.4 Some data derived from clinical studies in which patients with an invasive ESBL-producing enterobacterium have been treated with fourth-generation cephalosporins showed an unfavourable outcome when compared with patients treated with carbapenems.25
To the best of our knowledge, data on the in vitro susceptibility of Enterobacter spp. in Austria are scant,26,27 both for the frequency of occurrence of diverse AmpC producers and for ESBL-producing strains. Thus, in the case of empirical therapy, we would have to rely on data derived from other institutions, some of which report up to 30% ESBL-producing Enterobacter.8 According to this assumption, first-line empirical Enterobacter therapy would have to rely on carbapenems in any case.
The median number of blood cultures analysed at the Department of Clinical Microbiology of the Vienna General Hospital, a 2200 bed university-affiliated, tertiary care hospital with 242 intensive and intermediate care unit beds, is over 15 000 per year, with a mean positivity rate of 12%. Besides E. coli and Klebsiella spp., E. cloacae was the Gram-negative organism most frequently isolated from positive blood cultures between 1998 and 2005, causing 2700 bacteraemia episodes during this 8 year period. Retrospective review of all blood cultures positive for E. cloacae revealed that resistance rates for cefotaxime, which was tested during the whole period, increased from 11% in 1998 to 96% in 2002. As a consequence and independent from their phenotypic susceptibility pattern, broad-spectrum second- and third-generation cephalosporins were reported as clinically not indicated from 2003 onwards in the case of in vitro susceptibility. In 2005, 65% of Enterobacter bloodstream isolates were resistant to cefotaxime. Knowledge of the current situation with respect to resistance data, however, is of critical importance in our institution, since empirical Gram-negative therapy is largely based on third- and fourth-generation cephalosporins.
Although susceptibility to third-generation cephalosporins was evidently not given in partly and fully derepressed AmpC producers (94 strains; 46%), it would have been falsely assumed in 92 isolates (44%), which were in vitro susceptible to third-generation cephalosporins, but fully inducible AmpC producers. Thus, our strategy to report third-generation cephalosporins in Enterobacter isolates as being clinically not indicated was—retrospectively viewed—on the safe side. In addition, partially derepressed strains would have been missed as AmpC producers if they were tested in the absence of the inducer cefoxitin.
According to current CLSI breakpoints for Enterobacteriaceae, all isolates would have been susceptible to cefepime, irrespective whether they were derepressed, partially derepressed or inducible AmpC producers with and without additional ESBL production. The MIC90 of cefepime was similar in ESBL-producing derepressed AmpC producers (4 mg/L) when compared with strains without ESBL production (2 mg/L) and was especially low for inducible AmpC producers (0.125 mg/L). Detection of an inducible AmpC producer might give an additional hint to an especially low MIC for cefepime. In addition, a recently published review article indicates that the use of cefepime to treat serious nosocomial infections (e.g. bacteraemia, pneumonia and urinary tract infections) may be associated with clinical success, even in ESBL-carrying Enterobacteriaceae.28
Jacoby et al.29 have recently proposed a similar disc-based scheme that could be used to detect various ß-lactamases in E. coli and K. pneumoniae. Application of combined multiple double disc tests integrated in the routine antibiogram seems to us an easy and reliable approach to do so, especially in AmpC carriers such as Enterobacter spp., which might act as hidden ESBL reservoirs. Incorporation of these double disc tests into the antibiotic testing panel for Enterobacteriaceae saves time and financial resources. There were only eight ESBL producers in Enterobacter spp., which currently seem to play a minor role in our institution. It is, however, important to continuously monitor emerging resistance mechanisms on a routine basis since it may be difficult to draw this conclusion with this small number of isolates. In contrast to what others reported,15 Etest strips did not provide additional information when compared with disc-based tests in this study.
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We have no conflicts of interest or financial support to disclose.
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Acknowledgements |
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The excellent technical assistance of Cornelia Üblauer is deeply appreciated.
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References |
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29
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